For those of you who are new to this stationery thing, here is your little "learn something new everyday" moment. Hand cancelling an invitation is simply when the post office hand stamps each envelope with a post mark instead of running it through a machine which does this automatically. Why do this you ask? Well, there are several reasons... First a foremost it is an added personal touch for your special event, you put so much tought into the contents inside the envelope why stop there?! A more practical reason is because this takes away the possibility of one of the invites getting ruined running through the machine, which if your invite is overly thick it may have to be hand cancelled anyway. Lastly it eliminates all of the other "not so elegant" marks the postal machine puts all over your envelope.
Yes, I have heard that some post offices may refuse to hand cancel invites upon request, but I have also heard that if you go into your local post office at a slower time that you should have no problem. I'd say it is certainly worth a try, think how they will look to your guests when they receive it in the mail. Just another special added touch that will remind them how lovely and elegant your wedding was or is going to be!